1. Field of the Invention
The present invention relates to a touch probe which carries a surface contacting stylus, and is used on a coordinate positioning machine to sense the position of, for example, a workpiece surface. More particularly, the invention relates to a touch probe on which a number of different styli may be interchangeably supported.
One type of known touch probe is a "touch-trigger" probe. Touch-trigger probes emit a trigger signal upon sensing contact between a stylus carried by the probe and a surface. Typically, such a probe comprises a fixed structure, such as a housing, by which the probe may be supported on the movable arm of a coordinate positioning machine (such as a coordinate measuring machine or machine tool), and a stylus-supporting member. The stylus-supporting member is supported within the housing in a rest position from which it may be displaced when a deflecting force is applied to the stylus as a result of contact between the sensing tip of the stylus and the surface of a part to be inspected (this is known as "overtravel"). A biasing force is provided to bias the supporting member into the rest position, so that irrespective of the orientation of the probe, upon removal of the deflecting force the supporting member returns precisely to the rest position. Such a probe is known from U.S. Pat. No. 4,153,998. Another type of known probe is a "measuring" or "scanning" probe, which has a stylus carrying member suspended by a mechanism providing controlled deflections of the carrying member in 3-dimensions relative to the fixed structure of the probe. Transducers in the probe measure the magnitude and direction of these deflections relative to a reference point on the fixed structure (which is in turn fixedly connected to the movable arm of the machine). As a crash protection feature the stylus is retained on the carrying member by a biasing force, which biases the stylus into a repeatable rest position relative to the carrying member. The stylus may thus be displaced (in opposition to the biasing force) from its rest position on the carrying member in the event of a deflection of the stylus larger than that permitted by the suspension mechanism, thereby avoiding damage to the stylus. Such probes are known from WO89/05435.
It is frequently necessary to inspect parts of a relatively complex shape, having surfaces which are oriented in a variety of directions. One way to perform such inspection is to provide a probe with a suitably shaped stylus, which has one or more sensing tips oriented in such a way that the surface of the part to be inspected may easily be contacted. In order to provide a truly flexible measuring system however, it is necessary to provide automatic changing of one or more such styli on a probe.
2. Description of Related Art
Such an automatic stylus-changing system is known from, for example, U.S. Pat. No. 4,637,119, in which a magnetic clamping device is provided on the stylus-supporting member to enable releasable clamping of a plurality of different styli to the stylus-supporting member.
A problem with touch probes having stylus-changing systems is that different lengths of styli ideally require different biasing forces (a) due to the different weights of their differing lengths and (b) to prevent overloading of short styli. Manual adjustment of the biasing force each time a stylus is changed is impractical since such a stylus changing system is ideally automated to provide flexible part inspection. Automatic adjustment mechanisms for the biasing force (such as the one shown in U.S. Pat. No. 5,024,003) are complex and undesirably add to the weight of the probe.